Method for metalizing vias

ABSTRACT

A method for producing plated-through holes in printed circuit boards and to printed circuit boards produced in this manner.

The invention relates to a method for producing vias in circuit boards and to circuit boards produced in this way.

Ceramic circuit boards often need to be electrically contactable from both sides and to allow for through-plating through the circuit board. In general, this is carried out by means of holes that are provided in the circuit board and are filled with an electrically conductive material. Such vias normally have diameters of approximately 100-300 μm.

What is nowadays usually used as an electrically conductive material are comparatively expensive silver or other precious metal pastes which are composed of one or more metal powder(s), optionally an adhesive glass proportion of 1-10% by weight, for example PbO, B₂O₃, Bi₂O₃ or SiO₂, and a high-boiling organic matter, comprising is binders such as ethyl cellulose or polyvinyl butyral, and solvents such as Terpineol or Texanol. If the substrate material for the circuit boards consists of AlN, then ZnO, SiO₂, CaO, TiO₂ and B₂O₃ may be used as adhesive glasses.

Copper metallisations are also increasingly used as electrically conductive materials, wherein pastes filled with copper particles having diameters of approximately 1-10 μm and an adhesive glass proportion are often used for filling the vias. Subsequently, the material is burned in at 650-1200° C. in a nitrogen atmosphere having a low oxygen content (<1-100 ppm). If copper pastes are used, the problem arises that they greatly shrink, crack and/or fall back out of the vias during burning-in if they only consist of the usual substances mentioned above. In this case, large cavities and/or cracks may occur in the metallisation or in the binding region of the full-area metallisation. In the worst case, the vias may sinter together to form a pin and fall out of the hole. In cases in which cracks or cavities occur, the vias have an elevated electrical resistance and are not hermetically sealed either.

Tightness against the surrounding atmosphere is particularly important if oxygen-sensitive parts such as circuits (Si), LEDs or OLEDs are to be integrated. Towards the top, these components are usually protected by covers, lenses or the like, which are soldered or glued on. However, also the bottom has to be protected against the ingress of air or humidity, which is realised, inter alia, by means of hermetically sealed metallisations of the vias.

Attempts to counteract shrinking of the electrically conductive material during sintering are made by adding adhesive glass. Despite the adhesive glass contained in the paste, however, the negative effects of shrinking can often at least not sufficiently be avoided.

This phenomenon is a particular problem in the case of AIN ceramics because only few substances do not cause the decomposition of the AIN in heat. AIN is only metastable and decomposes forming the lowest energy aluminium compound Al₂O₃ under certain conditions. Some oxides, such as in particular the Bi₂O₃ that is often used, or the PbO that used to be applied, vehemently attack AIN, decomposing same to form aluminium oxide, lead and nitrogen according to the following reaction equation:

2 AlN+3 PbO→Al₂O₃+3 Pb+N₂.

Other oxides such as ZnO decompose AlN only slowly, and therefore they are better suited as binders of the metallisation to ceramics. In addition, it is important in the case of AlN to achieve a low TCE (thermal coefficient of expansion) such that the compound does not crack.

Moreover, conventional additives such as lead oxide or bismuth oxide are to be avoided for ecological reasons, or they react too strongly with AlN. The nitrogen that is formed from the AlN as a result of the decomposition reaction, see above, makes the metallisation porous and causes it to swell.

The problem addressed by the invention is therefore that of providing a method by means of which vias that are held securely in the holes of the circuit boards and have good characteristics in relation to current conduction can be produced. The method provided should be based on conventional methods as far as possible in order to avoid cost-intensive retrofitting. Moreover, materials that allow the production of high-quality vias at low costs are to be provided.

The problem is solved by a method according to claim 1 and a metallisation according to claim 12. The dependent claims define preferred embodiments of the invention.

Accordingly, a method for producing electrical vias, which avoids or in any case substantially reduces shrinking of the pastes during burning-in, comprises the following steps: mixing a paste, introducing the paste into holes of a circuit board and curing the paste under the effect of heat. Here, the paste comprises at least one electrically conductive material and fillers, wherein the fillers experience an increase in volume during curing, such that any volume shrinkage of the electrically conductive material is compensated by the curing under the effect of heat.

The invention is in principle suitable for circuit boards of any material, however, particularly preferred are circuit boards made of ceramics, in particular of a ceramics on the basis of AIN or Al₂O₃.

As an electrically conductive material, preferably copper particles, in particular having an average grain size d50 between 1 and 10 μm, may be used. Other conductive substances, in particular metals or metal alloys that are customary for these purposes, may also be used.

The fillers according to the invention may, for example, be nitride-forming substances that are added to the paste. Whilst being burnt-in under a nitrogen atmosphere, these substances or fillers nitrate and experience an increase in volume as a result of the nitration. This increase in volume compensates for reductions in volume of the conductive material, for example, by sintering the particles, at least to a large extent.

Such nitride-forming substances may in particular be aluminium, titanium, zirconium and/or mixtures of these substances. Preferably, these materials are added in amounts of 1-10% by weight, preferably 2-5% by weight, of the mixture for producing the pastes.

Further substances which experience an increase in volume during curing or conversion under the effect of heat may be swelling non-metals, in particular clays. According to a preferred embodiment of the invention, the swelling non-metals may constitute between 1 and 10% by weight, preferably between 2 and 5% by weight, of the mixture for producing the pastes.

Swelling clays comprise, for example, bentonite or montmorillonite which moreover advantageously have a high affinity to AlN or Al₂O₃ ceramics. A general affinity of the fillers to the ceramics used is generally desirable in order to ensure secure hold of the metallisation or of the electrically conductive material in the holes of the circuit board. With regard to the invention, affinity is understood as follows: the capacity of the electrically conductive material to form a solid bond with the material of the ceramics, which bond also does not immediately break under mechanical stress. The term “bond” is not only understood as a chemical compound, but generally as the durable joining of substances.

Each of the substances or fillers mentioned can, alone or in combination with others, compensate for the volume shrinkage of the electrically conductive material.

The mentioned metals that are added as fillers show, like the clay, an increase in volume during nitride formation, which compensates for the shrinkage of the electrically conductive material, in particular of the copper.

According to a preferred development of the invention, the adhesion between the metallisation and the circuit board substrate can be improved by means of oxides that react only slowly with the substrate material. If the substrate material consists, for example, of AlN, then ZnO, SiO₂, CaO, TiO₂ and B₂O₃ can be used as oxides that react only slowly with the substrate material. Moreover, also CuCl may be added as an adhesion-promoting substance that forms elemental copper under the effect of heat.

Moreover, adhesive glasses may additionally be added. In an Al₂O₃ ceramic substrate, adhesive glasses containing Bi₂O₃ are preferably used. These glasses have a thermal coefficient of expansion (TCE) that prevents the formation of cracks caused by stresses during the curing of the paste. Instead or in addition, adhesive glasses containing ZnO may also be added.

In the case of AlN ceramics as circuit board substrate, adhesive glasses containing ZnO are preferably added. However, adhesive glasses containing Bi₂O₃, as are preferably used in ceramic substrates containing Al₂O₃, are not particularly suitable for AIN ceramics because these glasses bond poorly with AlN, and their tensile stress may lead to cracks in the AlN ceramics.

A general formulation for a metallisation may be as follows:

-   -   oil, thixotropic: 5-20% by weight, preferably 8-12% by weight;     -   oil, screen printing medium: 5-20% by weight, preferably 8-12%         by weight;     -   electrically conductive material: 10-89% by weight, preferably         50-80% by weight;     -   fillers, total: 2-25% by weight, preferably 4-15% by weight, of         which         -   nitride-forming substances, in each case: 1-10% by weight,             preferably 2-5% by weight;         -   swelling non-metals, in each case: 1-10% by weight,             preferably 2-5% by weight;         -   adhesion-promoting substance, in each case: 0.1-5% by             weight.

The invention will be explained in more detail below by way of examples. The examples are merely intended to explain the invention and are in no way to be regarded as limiting the invention.

A paste for metallising vias in an AlN or Al₂O₃ ceramic substrate is produced according to the following recipe:

oil, THIXOTROPIC 10% by weight oil, screen printing medium 10% by weight copper powder, d50 = 10 μm 1-90%, preferably 50% by weight copper powder, d50 = 1 μm 0-50%, preferably 12% by weight aluminium powder, d50 = 8 μm 0.1-10%, preferably 5% by weight bentonite, d50 = 12 μm 0.0-10%, preferably 5% by weight zinc oxide, d50 = 5 μm 0.1-5%, preferably 2% by weight quartz flour, d50 = 6 μm 0.1-5%, preferably 1.5% by weight boron oxide, d50 = 10 μm 0.1-5%, preferably 1.5% by weight copper(I) chloride, d50 = 5 μm 0.1-5%, preferably 3% by weight

The solid weighed components of the paste are mixed in a tub using a spatula. Subsequently, the weighed oils are added. As thixotropic oil, HERAEUS 212/thix may preferably be selected, which is liquefied using HERAEUS 213 oil. The substances are coarsely mixed with a spatula and then slowly (approximately 1-2 kg/min) run through a three-roll mill (for example, EXAKT or the like) twice.

A ceramic substrate made of AlN and having a thickness of 0.5 mm is used; the vias to be filled have a diameter of 0.15 mm. The paste is filled into a screen having openings directly above the vias, i.e. the holes to be filled in the sintered ceramic substrate. The paste is then brushed into the vias using a plastic blade. The vias may have been punched in a raw state or laser-cut in a burnt state. Low sintering glass ceramic substrates (approximately 900-930° C.) may also be filled in a still vitric state and subsequently ceramicised with the paste filled in. The grain sizes of the components of the paste should preferably not exceed 20 μm, otherwise they could clog up the conventional screen fabrics and may have insufficient reactivity. This can lead to undesired cavity formation or to too few reactants in the vicinity.

In order to introduce the paste into the holes of the ceramic substrate, screen printing methods are preferred, since the circuit boards are frequently printed using screen printing methods, and therefore this type of application can be realised in an efficient and simple manner. In principle, however, also any other methods are possible by means of which the holes of the circuit board can be filled with the paste.

Burning-in is carried out at temperatures between 800 and 1000° C., preferably at 900° C., under a nitrogen atmosphere with a low oxygen partial pressure and was carried out in the example mentioned above at 900° C. for 10 min at maximum temperature; in the example mentioned above, the oxygen partial pressure was 10 ppm.

The result shows that the vias are well filled, are slightly porous and the copper adheres to the AlN ceramic. The metallisation closes a hole in the circuit board in a hermetically sealed manner. The helium leak test according to DIN EN 1779:1999 results in 10⁻⁸ mbar/l*sec.

By comparison, a paste without additives, just with copper, i.e. without aluminium and bentonite, was filled into the vias of an AIN substrate and burned-in.

The metallisation shows large cavities and does not adhere to the wall in some places. The electric resistance is, due to cracks, 30% higher than in the example above, the viasare not hermetically sealed (helium leak test >10⁻⁶ mbar/l*sec). 

1-15. (canceled)
 16. A method for producing electric vias in a circuit board, comprising the steps of: mixing of a paste; introducing the paste into holes of a circuit board; and curing the paste under the effect of heat; wherein the paste comprises at least one electrically conductive material and fillers; and wherein while curing, the fillers experience an increase in volume, such that a volume shrinkage of the electrically conductive material is compensated by the curing under the effect of heat.
 17. A method according to claim 16, wherein a circuit board made of ceramics, in particular of ceramics on the basis of AlN or Al₂O₃, is used as a circuit board.
 18. A method according to claim 16, wherein the fillers constitute 2-25% by weight, of the overall mass of the paste.
 19. A method according to claim 16, wherein materials that can be nitrated experience an increase in volume during nitration are used as fillers.
 20. A method according to claim 19, wherein the fillers comprise aluminium and/or titanium and/or zirconium.
 21. A method according to claim 16, wherein the fillers comprise swelling non-metals which swell under the effect of heat and thus experience an increase in volume during curing.
 22. A method according to claim 16, wherein the swelling non-metals comprise clays.
 23. A method according to claim 22, wherein the clays are selected in such a way that they have an affinity to the ceramics of the circuit board.
 24. A method according to claim 20, wherein the clays comprise at least one member selected from the group consisting of bentonite and montmorillonite.
 25. A method according to claim 16, wherein in addition, adhesion-promoting substances are added which enhance the affinity of the paste or the metallisation to the substrate of the circuit board.
 26. A method according to claim 25, wherein the adhesion-promoting substance is an oxide selected from the group consisting of ZnO, SiO₂, CaO, TiO₂ and B₂O₃.
 27. A method according to claim 25, wherein the adhesion promoting substance is CuCl.
 28. A method according to claim 16, wherein the paste is introduced by means of a screen printing method.
 29. A circuit board with a metallized via the process the method according to claim 16, comprising at least one electrically conductive material and fillers, wherein the metallization hermetically seals a hole in a circuit board.
 30. A circuit board according to claim 29, wherein the helium leak test according to DIN EN 1779:1999 results in values of less than 10⁻⁷ mbar/l*sec for the via. 